The EtherCAT diagnosis is a feature of the EtherCAT master. Placing add-ons on different levels of the model tree wouldn't help. All the diagnostic information the add-on displays in the HMI comes from the master, and the master handles all slaves as a flat list, ignoring the bus hierarchy.
Apart from that, I am not sure if it makes sense to show only part of the bus errors. I would expect that errors from one part of the bus can affect the rest of the bus, i.e. all stations. (But I must admit I am not an EtherCAT expert.)